uhh I think you must be thinking of Rabies, that is the only virus that cannot be tested for until after death.

FIP is a coronavirus (similar in nature to the Corona and Parvo viruses that dogs get) & testing will give a definative diagnosis.

Actually, Remy21 is right about how difficult it is to test for FIP. You see, FIP is a mutation of the coronavirus, which many 'normal' cats have anyway, so the presence of FCoV antibodies is not a definitive diagnosis. Here's some more info from a leader in FIP research: http://www.dr-addie.com/WhatIsFIP.ht...sis%20of%20FIP

Quote:

Diagnosis of FIP – this section is intended for veterinary surgeons
FIP is a notoriously difficult condition to diagnose, many other conditions present with very similar clinical signs. Definitive diagnosis is only possible at post mortem, or occasionally by biopsy (though for accurate biopsy results one has to actually biopsy a visible pyogranulomatous lesion, which may necessitate laparotomy). Only 18% of samples sent to our laboratory for FIP diagnosis turn out to be FIP. Since cats with FIP are usually euthanased, it is absolutely vital that FIP is accurately differentiated from other, treatable, conditions.

In our laboratory at the University of Glasgow, we offer an FIP profile which confirms or rules out a diagnosis of FIP in over 90% of cases. The FIP profile consists of four parts: a feline coronavirus (FCoV) antibody titre, albumin:globulin (A:G) ratio on the effusion or plasma, alpha 1-acid glycoprotein (AGP) level and cytology or haematology.

Note: many healthy cats and cats with diseases other than FIP have FCoV antibodies. The presence of FCoV antibodies alone is NOT diagnostic of FIP, if the other parameters of the profile do not indicate a diagnosis of FIP.